This invention relates to telephone line circuits and to battery feed circuits for such telephone lines.
In a telephone subscriber loop comprising a telephone line a subscriber subset and central office circuits it is necessary to supply a dc voltage in order to provide talking current. It is also necessary to provide some means of reducing noise signals which are induced onto the telephone line and are characteristically common mode signals in contrast to the differential mode talking signals. Prior battery feed, common mode interference reduction circuits comprise one large inductor with two windings, one connected from a first wire of the telephone line to ground and one connected from a second wire of the telephone line to battery. The two windings are closely coupled whereby a high impedance is presented to differential signals and a low impedance to longitudinal signals. Although the inductor is relatively inexpensive it is physically bulky and therefore results in large secondary costs in the area of packaging. It is desirable to implement a telephone line battery feed circuit using integrated or discrete circuit technology rather than the prior discrete inductor.